Shift Shaft Seal replace

'87 truck 727, shift shaft seal is leaking, mainly when sitting. How big of a job is this, and does anyone have any tips to go by here? Is my daily driver, hoping to be able to get this done in a day or 2 assuming no suprises (we all know how that is lol). I'll add that i'm a complete noob to transmissions. TIA fabo!